Summary.The lower air
resistance of aero wheels compared with conventional spoked wheels
will reduce your time in most road events by a valuable 2%. At the
slower speeds of most off-road events, the difference in air
resistance has a negligible effect on performance. Aero wheels also
weigh more, cost more, and most can't be trued. My recommendation:
use conventional spoked wheels, unless you race competitive downhill
or cross country time trials.Reviewers' comments

Aero wheels are
nothing new. In 1891 the British publication Cycling
considered disk wheels the "greatest invention to affect the modern
bicycle", but they began to appear in competitive road racing only a
decade ago. Manufacturers of aero road wheels, including Spinergy,
Spin, Zipp, and Hed, have now designed models to be used by the fat
tire set. Several influential off-road race teams are sponsored by
these companies, increasing the exposure of aero wheels designed
specifically for mountain bikes. Do they work on trails? There isn't
much published data, but what there is, and some simple physics,
makes it reasonably clear that aero wheels are worthwhile in the
faster off-road events.

What's the big deal with aero wheels anyway? You probably know the
answer already. The wire spokes on a normal wheel experience a
resistive force as they slice through the air, just like the wires of
an egg whisk in a bowl of water. An aero wheel effectively reduces
the number of spokes and the drag of each spoke. The drop in
resistance is enough to give you a valuable increase in speed of a
few percent in road events. But as we'll see, the gains off-road are
generally less because of the slower speed of these events.

LiteratureReviewed are contents from the
Zipp Components and
Spinergy commercial
websites, the Mountain Bike Review
website, a study reported in the European Journal of Applied
Physiology in abstract form only (Capelli et
al., 1993), a classic paper on energy expenditure in cycling
(di Prampero et al., 1979), and an article in
Inside Triathlon(Zinn, 1995).The
BikePro website provided data
for wheel prices. The published data are for the effects of air
resistance on road bikes and road wheels; I've had to resort to
physics to adapt these findings to off-road races.

FindingsThe advantage of aero wheels in road races is undisputed.
According to a mathematical model devised by Jim Martin, Director of
Sports Science for Team EDS, an elite cyclist can expect to complete
a 40-km individual time trial in 55:57 with conventional spoked
wheels in calm conditions and excellent body position. With aero
wheels (the Specialized Tri Spoke in front and a lenticular rear
disk), the time saving is 1:26. The faster time equates to an average
speed of 44 km per hour. Under various conditions, the time
savings with aero wheels in the model are consistently 1-2 minutes
over a distance of 40 km, equivalent to 2% for a slow rider and
closer to 3% for elite cyclists (Zipp
Components). In a study with real bikes and riders, Capelli et
al. (1993) reported savings of 3% over
distances ranging from 1 to 20 km when aero wheels replaced
conventional spoked wheels. These results fit nicely with the
increase in world-record speeds of about 4% that occurred after the
adoption of the new aero bikes. Most of this increase is due to the
aero wheels; the rest comes from improved aerodynamics of the frame
and handlebars.

Aero wheels were originally disks, but they are now also made with
aero spokes. Lennard Zinn (1995), technical
writer for Inside Triathlon and a well-known frame builder,
claims disk wheels perform best under nearly all conditions and even
offer negative drag readings at extreme wind angles (negative drag
means the disk wheel propels the bike forward). The downside is that
disk wheels pose handling problems when you ride in 90 degree
crosswinds, so they are unsuitable for group riding and mass-start
races. New aero spoke wheels have virtually eliminated this
disadvantage while providing almost the same time savings during an
event.

The table below, from Zinn's article, shows that for a given
relative wind angle, spoked aero wheels perform nearly as well as a
disk. Relative wind angle is the direction of wind experienced by the
rider (and bike). Relative wind angles of greater than 15° are
rare, even in a strong 90° crosswind, because the bike is
usually going a lot faster than the wind. Spoked aero wheels fare
well, but a disk wheel pulls away from all challengers when the wind
is angling in.

Two factors come into play to reduce the possible advantage of
aero wheels for off-road racing: the reduced speed in most off-road
events, and the greater weight of aero wheels. The difference in
speed is the crucial factor.

Off-road events are usually slower, for a number of reasons. In a
road time trial, a cyclist assumes a tight aerodynamic position, but
in a mountain bike event, most riders spend a large portion of the
course using aggressive body English to negotiate the technical
sections. Air resistance from the rider is therefore greater in
off-road events. The rolling resistance of mountain-bike tires also
greatly exceeds that of a smooth road-bike tubular tire (although
tires designed with relatively smooth tread have begun to appear in
Europe). Roughness in the off-road surface adds even more to the
rolling resistance. All these factors equate to slower average speeds
in off-road events. Hilly courses are also slower if the rider has to
brake on the downhill sections, and the variations in power output in
a hilly course result in lower average power output and speed, for
physiological reasons I won't go into. But these are all usually
minor factors. The main reason for the slower speeds in off-road
racing is simply that the courses don't give the rider the
opportunity to maintain high speeds.

How does slower speed affect air resistance? Drag force increases
in relation to the square of the speed. In other words, when
you double your speed the drag force increases fourfold, because
"twice as many air molecules collide with you and your bike twice as
fast" (Burke, 1995). But the energy expended
against a resistive force is also directly related to the speed so
the overall effect of air resistance is proportional to the
cube of speed. Let's turn this around and think about the
effect of going slower. If your gain with aero wheels in a 40-km time
trial is 2%, and you do the time trial at 40 km/hr, then your savings
in an off-road race at half the speed will be only 2%/8, or about
0.25%. That's assuming the aerodynamic characteristics of off-road
and on-road aero wheels are the same. There are no data on this
question.

These savings need to be adjusted in various ways. If the speed
varies a lot around 20 km/hr, air resistance will contribute
relatively more at the faster speeds, and the savings might go up to
0.4%. But this figure was reached by assuming that an off-road race
is just like a slow on-road race. It's not. Arnie Baker pointed out
in his first reading of this article that riders often "float"
technical sections of off-road races, not braking nor pedaling. At
such times the rider isn't working against air resistance so the gain
might be 0.2-0.3%. And this gain is for 20 km/hr. Most off-road
events are a lot slower. Last weekend, the race I was at had the pros
turning 8 km lap times every 35 minutes, averaging less than 15
km/hr. At that speed, aero wheels simply won't help.

Speeds for some events are increasing. Recently there has been a
proliferation in the number of off-road stage races. These events are
patterned after their road counterparts and usually consist of an
off-road time trial, a circuit style criterion, and a "normal"
off-road race thrown in for good measure. In the 1997 Sea Otter
Classic, Hubert Pallhuber (Diamondback) won the 7.7 km time trial
stage in 14:23:7, giving him an average speed of 32 km/hr. At that
speed, aero wheels could be a small benefit. Downhill events show
much more promise for aero wheels. At certain events such as Mammoth
Mountain, cyclists have been clocked at speeds exceeding 90 km/hr. At
speeds this high, aerodynamics play a very important role.

Now, what about the weight factor? Weight is crucial in cycling.
In spite of the availability of full suspension bikes, nearly all
professional riders still choose to ride on hardtails, which have
only front suspension. The reason? A full suspension bike weighs too
much. Aero wheels are heavier than normal wheels, but how much
heavier? Although Spinergy does not claim weight for their rear
wheel, they do mention that their front wheel weighs 895 grams. A
standard spoked wheel consisting of a Shimano XT front hub laced to a
32 hole Mavic 217 SUP rim with DT 14/15 gauge spokes weighs 794
grams, including skewer. This equates to a difference of 101 grams,
or almost a quarter of a pound. If a more exotic front wheel is
desired, a Hershey Naked hub can be substituted. The weight
difference then rises to 239 grams, or over half a pound.

The effect of this extra weight of aero wheels on performance is
less than you might think, according to my consultant
physicist. If we assume a rider weighs 65 kg and a bike weighs 10
kg, the extra weight represents about 0.3% of the combined weight of
the bike and rider. When you go up a steep climb, it will take you
0.3% longer because of the extra weight. Coming down again, you could
go this much faster with the extra weight, and you'll recover the
lost time if you don't have to brake. For hills that aren't steep,
the loss in time is less than 0.3%. On the flat, the extra weight has
no effect, provided you don't change speed. But every time you
accelerate, it will take you about 0.5% longer to reach a faster
speed (not 0.3%: see note). Again, you will
recoup the lost time, provided it's the course that slows you down
after the burst of speed; if you have to brake to slow down, you
won't make it up. It's often the hills that slow you down, not just
the next technical bit that you have to brake for, so I would guess
that the loss of time due to the weight of aero wheels is maybe 0.4%
of the time spent accelerating and 0.2% of the time going up hills.
And how much time is spent going up hills or accelerating?
Let's say a quarter of the race for each, which makes a total loss of
time of 0.15%. Negligible. I find it hard to believe, but there it
is. In spite of calculations by my consultant physicist that prove
the insignificance of the weight argument,many off-road
racers who depend on their ability to climb will not tolerate an
increase of any magnitude. Although an increase of just a percent or
two may seem small, the mental aspect of thinking that you are adding
weight by using aero wheels (even if it is just a little) may cause
you to slow down. Never discount the psychological factors!

A final consideration is the durability and cost of aero wheels.
The rigors of racing off-road increases the potential for broken
components. Aerodynamic wheels are considerably more expensive than
their spoked counterparts. A pair of Spinergy wheelscosts
US$800. In comparison, the pair of Hershey hubs, Mavic 217 rims and
DT 14/15 gauge spokes costs $420. So the Spinergy wheels would have
to last twice as long to be cost effective. Unfortunately, it is not
possible to true most aero wheels, so the lifespan may be shorter,
especially if the rider uses them for downhill racing. Spinergy and
Spin both offer replacement policies for a tweaked wheel at a
significantly lower cost than a new aero wheel--about half the price
of a new wheel, including postage, but still far more than having
wheels trued at a bike shop.

Ultimately, cyclists will decide if aero wheels are worthwhile. At
the Mountain Bike Review website,
the section providing comments on
Spinergy
wheels is split roughly 50/50, with half the comments raving
about how great these wheels are and the other half condemning them.
Most reviewers do agree that the wheels flex considerably when
riding. Some riders feel that this helps provide a cushion effect
which lets them ride longer with less fatigue, while others feel that
this motion is disconcerting and actually saps energy.

ConclusionsIt's fairly certain that the aerodynamic benefits of aero
wheels designed for mountain biking are limited to fast races. Expect
no gains in speed in normal off-road races, but worthwhile benefits
of a percent or more as speeds get close to road speeds. And for
downhill races, aero wheels seem certain to give you a healthy lead.

Further ResearchMore specific tests designed with mountain bike conditions
need to be done to determine how pronounced the aero benefits are for
these wheels.According to information presented at the
Zipp Components website, the lack
of "established standards" poses serious problems when conducting
aerodynamic studies. Several manufacturers who are conducting
internal testing may be guilty of biasing their own results to ensure
that their product appears in a favorable light. Studies that are
conducted under the supervision of third party examiners need to be
performed. These results then need to be published regardless of
which set of wheel performed the best. Roll down tests could be
performed, as well as real studies and mathematical modeling that
includes hills and the effects of weight and rolling resistance
(rotational inertia).The results of this research ideally
would be guidelines for cyclists to determine whether they would gain
by using aero wheels for a particular race.

Zinn, L. (1995). Why are disc wheels so rare in
triathlon? Inside Triathlon, September 1995, 50.
Estimates of the effects of changing speeds and increased weight
(including the note below) were provided by
Will Hopkins.
Note: When you accelerate, an increase in mass of a wheel by 0.3%
slows you a bit more than 0.3%, because you give the wheel rotational
energy as well as linear energy. It depends on where the extra mass
of the aero wheel is distributed. If it's all around the rim, add an
extra 0.3%. If it's all at the hub, there is no extra effect. Let's
say 0.5% for aero wheels.

I agree with the author's conclusion: aerodynamic wheels can't
have meaningful aerodynamic benefits for mountain biking, because air
resistance plays a much smaller role in the slower off-road races.
The wider mountain-bike rim may also shroud the spokes, which would
reduce the effects of air resistance and thereby reduce even further
the saving of aero wheels relative to normal wheels.

The author correctly identifies that weight is an issue. As
important as the climbing issue, which the author notes, is the fact
that mountain biking is a sport of surging--large power fluctuations.
Heavy wheels do not accelerate well. Dean Golich, a sports scientist
with USA Cycling at the time, studied cross-country riders on the
Atlanta course prior to the 1996 Olympics. Using an SRM crank (a
device for recording power developed by the cyclist), he found almost
continuously repeated power fluctuations over 100 watts. Although
road cyclists will use heavier wheels in relatively steady-state time
trials, they will not accept this weight penalty in criteriums, where
repeated surging out of corners is crucial to success.

The author raises the issue of crosswinds. Crosswinds can be a
factor in exposed downhill mountain runs, but are rarely an issue in
cross-country mountain biking. Handling difficulties are usually only
a problem with an aero front wheel, not an aero rear wheel.

The author states that aero wheels cannot be trued; that may be
true of Spinergy; but Zipp, Buzz and Hed aero wheels can be trued.
Flex may also be less of an issue with these wheels.

The lack of data points up a need for research to establish the
speeds at which air resistance starts to have an effect with
mountain-bike aero wheels. The mountain bike market is larger than
the road market; so it's logical that manufacturers will try to
exploit this market to the fullest.

Jason Nugent has identified the one bike component in which
mountain bike technology lags behind that of its older cousin, road
cycling. I agree with his conclusions that there are good reasons the
off-road wheel remains rather low-tech in an otherwise high-tech
sport, at least in the cross-country event. It's obvious to me that
at the speed of most off-road races--hovering around 20 km/hr at most
for top male riders-- the contribution of work done against air
resistance must be tiny compared with the work the rider puts out to
negotiate hills, change speed continuously, and overcome the rolling
resistance of the tires. In the downhill event, with speeds exceeding
80 kph at times, air resistance is exceptionally large and has a
definite impact on finish times. Aero wheels are an advantage here.

As explained by Jason, cost is an important consideration in a
sport that places great stress on equipment. Few non-sponsored riders
are willing to shell out the additional money to replace damaged aero
wheels when standard, spoked wheels can often be trued at a fraction
of the cost.

Editor's Note

Even one of the manufacturers' executives, Zipp's Bill Vance,
concedes that aero mountain wheels aren't going to make you fast at
typical mountain bike speeds. Vance does claim that the wheels will
help you accelerate more quickly and give you a more comfortable ride
and control over the bike. Perhaps instead of aero wheels, we should
call these "suspension" wheels and suspend usage until we get some
practical test results.
Mary Ann Wallace